Adds a mr-device-mounted binary to MondoRescue? (replaces the code of the C function is_this_device_mounted) and the corresponding perl function (mr_device_mounted) to handle cases where a mounted device has a different name than the one seen on mount (e.g. /dev/rhel/root and /dev/mapper/rhel-root)

Adds perl function mr_file_copy_and_erase_hash to empty the hash generated in mr_file_read_all_link and thus avoid filling the hash it generates, which is useful for a recursive function, but not when called multiple times from another perl script. To be improved later on.

Adapt build procedure to the new binary (tested on Mageia 5 and RHEL7)